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Development and Application of the First In-Service Inspection Machine for the Pressure Reactor Vessel in China Zhu Rong ( Research Institute of Nuclear Power Operation, P.R.China ) Contact

1. Introduction

Nuclear Reactor Pressure Vessel is an important equipment of a nuclear power plant The pre-service and in-service inspection of the pressure vessel is clearly demanded in ASME code, RSEM code and nuclear safety guide issued by Nuclear Safety Bureau of China. The inspection result is the evidence to testify the integrity of the reactor pressure vessel and one of the key items on the safety evaluation about nuclear power plants.
The nuclear reactor pressure vessel inspection machine is a complicated integrative electromechanic system. This system is a large remote control roboticized system consisting of high precision mechanical construction, electronic drag and control, advanced ultrasonic inspection technologies and computer technologies.

2. Inspection Code

According to requirements of the pre-service and the first in-service inspection programs of CHASIMA nuclear power plant in PAKISTAN, ASME XI (edition 89) has been adapted as the pre-service and in-service inspection code for nuclear safety level I,II and III components. R.G.1.150 Ultrasonic Inspection of Reactor Vessel Welds during Pre-service and In-service Examination has also been required in inspection of nuclear reactor pressure vessel. So, the pressure vessel inspection must meet to all the inspection requirements of ASME XI (edition 89) and U.S. Regulator Guide R.G.1.150.

3. Inspection Components

Nuclear reactor pressure vessel inspection machine automatically inspects the pressure vessel by remote-control immersion contact ultrasonic inspection technology. According to ASME XI (edition 89) IWB2000, the following parts should be detected:
• Shell Weld
• Bottom head weld
• Integral attachment weld
• Shell to nozzle welds
• Inlet to safe-end dissimilar metal butt weld
• Safe-end to main pipe butt weld
• Outlet to safe-end butt weld
• Safe-end to outlet butt weld
• Shell-inlet inside radius section
• Shell-outlet inside radius section
• Flange stud holes

4. Probe and scan scope

Six ultrasonic scanning tools are adapted to the above eleven parts, including:
• Shell weld scanning tool ( scan shell welds and integral attachment welds)
• Bottom head weld scanning tool ( scan bottom head weld)
• Nozzle weld tangent scanning tool ( scan junction weld between shell and nozzles)
• Nozzle core scanning tool (scan inlet or outlet to shell welds and nozzle to safe-end or safe-end to main pipe welds)
• Inside radius section scanning tool ( scan inside radius section of nozzles)
• Flange scanning tool (scan flange stud holes)

4.1 Shell weld-scanning tool
Eight(8) UT transducers were used in this scanning tool, including 70°, 45°, 0°and 60°.
There are two scanning methods:

• scanning direction is axis and raster direction is circumference
• scan direction is circumference and raster direction is axis

After the first scanning method, the probe plate is turned by 90°to do the second circumferential scan. These probes are also used to scan integral support attachment welds.

4.2 Bottom head weld-scanning tool
Eight(8) transducers were used in this scanning tool, including 70°, 45°,0°,and 60°.
The scanning methods are same with the methods of shell weld scan.

4.3 Nozzle core-scanning tool
This tool is used to test the nozzle to shell or safe-end weld. Twelve(12) transducers were used in this scanning tool, including 70°, 45°, 0°, 50°and 30°. There are two methods of probe scan. By the first method, scanning direction is circumference and raster direction is axis of the nozzle. By the second method, scanning direction is axis of nozzle and raster direction is circumference.

4.4 Nozzle weld tangent-scanning tool
Eight(8) transducers were used in this scanning tool, including 70°,45°, 0°and 60°.
The scanning direction is along circumference and raster direction is along axis of the nozzle.

4.5 Inside radius section scanning tool
Six(6) 70°transducers were used in this scanning tool.
The probes (70°, double-element, long wave, angle probe) are used to test the internal bend of inlet nozzle. Two probes (70 degree, single-element, long wave, angle probe) are used to test the internal radius section by T/R method, one is located at inner side and another is at outside. This technology has the functions of defect detection and sizing.
The scanning method is that scan direction along circumference and raster along axis. Attachment test of outlet internal radius section is done while the nozzle core scanning tool is working.

4.6 Flange scanning tool
Flange scanning tool is used to test the screw root of stud hole by hand. An Operator stand on the upper head to test the screw root of flange by manually rotating the scanning tool, after the upper head has been lifted up 500mm above the reactor pressure vessel.

5. Reliability and repeat of automatic ultrasonic inspection system

Automatic remote control system should be used to protect operators from the strong radioactivity in running nuclear power plant while periodic test should be taken during the nuclear power plant in service according to inspection code. So, reliability and repeat are very important to the inspection system.

5.1 Requirements for nuclear reactor pressure vessel ultrasonic inspection


• Automatic remote control (150m)
• The overlay field more than 50% element
• Repeat positioning precision better than ±5mm
• 100% volumetric test taken over welds and heat effect area
• To shell weld, bottom head weld, junction welds between nozzle and shell, shell support attachment welds, an indication more than 50% DAC detected by 0°probe should be recorded according to test standard, an indication not less than 20% DAC detected by angle probe should be recorded.
• An indication not less than 50% DAC should be recorded in the ligament of flange stud test.
• An indication not less than 50% DAC should be recorded in internal radius cross-section test.

5.2 Ultrasonic system
We have selected an ultrasonic equipment ( m+ 16-channel digital ultrasonic instrument made by U.K. AEAT) to meet the requirements of ASME XI (edition 89), R.G1.150 and the probes described in section 4. The capability of m+ is very advanced in the world, the sampling rate up to 64MHz, resolution up to 8 point, 8192 points of signal may be gotten in the gate. Meanwhile, we can use 13 probes a time to test the vessel. The software we used includes:

• Multi-channel pulse echo analysis software
• TOFD software
• TOFD assistant analysis software
• Frequency spectrum analysis software
• Report production software

All these software can meet the needs of data collection and analysis of the complete vessel test.

5.2.1 "Time of Flight Diffraction" (TOFD)
TOFD is a new ultrasonic inspection and data process technology from U.K. AEAT, which is much different to the normal ultrasonic inspection with pulse echo. The advantage of TOFD lies in that defect locating and sizing is independent to the defect direction and the amplification of the defect sign, the precision may up to ±1mm.

5.3 Methods to improve the test reliability


5.3.1 Ultrasonic probes
42 probes have been used according to ASME XI (edition 89) and R.G.1.150. Characters of all the probes meet the requirement of above code, each probe has a testing result report and a qualification license. A periodic calibration also used to make probes work properly.

5.3.2 calibration blocks for the first and the second calibration
According to ASME V(edition 89) article 4 T-440, we designed 5 calibration blocks with artificial reflectors for the first calibration, including:

• Shell and bottom head circumferential weld block
• Inside radius section block of inlet and outlet
• Dissimilar metal weld of nozzle block
• Block for nozzle tangent scan
• Block for flange stud hole scan

To insure the reliability of ultrasonic data in test circle, a calibration should be taken every 12 hours according to ASME code to confirm the sensitivity change of ultrasonic system. We also designed 4 blocks for the second calibration, including:
• Shell and bottom head circumference weld block
• Block for nozzle tangent scan
• Block for nozzle core scan (axial)
• Block for nozzle core scan (radial)

5.3.3 Coupling condition maintenance
Zero degree probe is used to monitor the coupling condition of scanning probes on the components tested. We also use the C-scan image to watch the coupling situation of probes during acquiring data.
In addition, constant pressure springs are used in probe plate to make probes tightly contact to the component surface, which ensure the coupling of probes and protect the signal amplification from changing by the pressure.

6. The mechanical construction and location precision of the automatic inspection system

6.1 System construction
Drawing 1 shows the construction of this system.

Drawing 1 : Construction of Reactor Pressure Vessel Inspection Machine

6.2 electronic control
Two different site operation has been considered in the electronic control design (including remote control in a container outside the safety shell and operation inside the safety shell). We use two industrial control computers as the console system including upper machine and lower machine.
The upper machine is located in the container, serving as the operation plate of inspection plan management and inspection execution by remote control , as well as a machine for character production and attachment. The lower machine is fitted in control tank, serving as the translator of movement orders and operation service, which can perform the inspection by itself when necessary. Communication of the two computers does by a RS-422 cable, using odd-even check and key words check to avoid miscodes. The serial communication is set under 115,200 baud rate.
The control objects of lower machine are two types of DC servo ( 6 servo sets altogether), signal inputs of photoelectricity isolation, all inputs of relay, decodes of encode signal and the interface of ultrasonic system.
The connection diagram is shown as following:

Fig: Electronic Control Connection Diagram

7. Creative technologies

• Motion parts of the system is shown with dynamic images
• Back-and-fro motion of manipulator by thread stick and glide block
• TOFD is used in defect location and sizing the first time in China.
• Compounding coordination signals of 6 axis and ultrasonic signals of 8 channels by pulse compensation method
• Designing and fitting device for compounding video signal and console coordination signal by ourselves
• Designing and producing three simulators for making function examination of scanner and evaluation.

8. Pre-service reactor pressure vessel inspection of PAKISTAN CHASIMA nuclear power plant

The PRV test completed in CHASIMA includes following items:
1. Longitudinal and circumferential inspection of flange-shell circumferential weld
2. Longitudinal and circumferential inspection of shell-bottom head circumferential weld
3. Inside radius section inspection of inlet-shell
4. Inside radius section inspection of outlet-shell
5. Longitudinal and circumferential inspection of outlet to safe-end butt weld
6. Longitudinal and circumferential inspection of inlet safe-end butt weld
7. Longitudinal and circumferential inspection of inlet safe-end to main pipe dissimilar metal weld
8. Longitudinal and circumferential inspection of outlet safe-end to main pipe dissimilar metal weld
9. Ultrasonic inspection of integral attachments
10. Ultrasonic inspection of flange studhole threads
11. Ultrasonic inspection of inlet or outlet to shell weld
12. CCTV inspection of shell cladding surface
13. CCTV inspection of inlet or outlet inside radius section
14. CCTV inspection of inlet or outlet internal cladding surface

The whole procedure was monitored by specialists from employee and PAKISTAN. All parts of the system are completely meet the requirements of ASME code. This system worked steadily, reliably, with high precision and complete functions. All ultrasonic indications were analyzed by locating and sizing with B, C, D scan.

9. Conclusion

This nuclear reactor pressure vessel inspection machine with auto-scanning ultrasonic system can meet the requirements of ASME XI and regulator guide R.G.1.150 in inspection scope, synthetic performance, position and repeat position precision, which has successfully completed the reactor pressure vessel inspection of CHASIMA in pre-service inspection, and will be used in the pressure vessel inspection of 300MW reactor units of Qinshan Nuclear Plant in China. Many technologies adapted in this system can also be used in the inspection machine for primary equipment and pipe auto-inspection of nuclear plant. Drawing 1.